adb: switch apacket payload to a type that doesn't initialize its contents.

Switch from using std::string as the type we use to hold our payload in
apacket to a custom reimplementation that doesn't zero initialize. This
improves bulk transfer throughput in the adb_benchmark microbenchmark
on walleye by ~20%.

Test: adb shell taskset f0 /data/benchmarktest64/adb_benchmark/adb_benchmark
Change-Id: Ibad797701eb1460c9321b0400c5b167b89b2b4d0
diff --git a/adb/socket_test.cpp b/adb/socket_test.cpp
index 6c4a8b2..04214a2 100644
--- a/adb/socket_test.cpp
+++ b/adb/socket_test.cpp
@@ -118,7 +118,7 @@
         // each write to give the underlying implementation time to flush.
         bool socket_filled = false;
         for (int i = 0; i < 128; ++i) {
-            std::string data;
+            apacket::payload_type data;
             data.resize(MAX_PAYLOAD);
             arg->bytes_written += data.size();
             int ret = s->enqueue(s, std::move(data));